This invention relates to image processing methods used in ultrasound medical diagnostic imaging systems. More particularly, this invention relates to a method for automatically measuring fetal head size from ultrasound image data.
Ultrasound medical diagnostic systems generate images of anatomical structures within a patient's body by scanning a target area with ultrasound signals. Typically, ultrasound signals on the order of 2.0 MHz to 10 MHz are transmitted into a patient via a transducer probe. The transmitted ultrasound energy is in part absorbed, dispersed, refracted, and reflected by the patient's body. Reflected ultrasound energy is received at the transducer probe where it is converted into electronic echo signals. The echo signals undergo beamforming to correlate the ultrasound signals. Subsequently the beam-formed signals are processed to analyze echo, doppler, and flow information and to obtain an image of the patient's targeted anatomy (e.g., tissue, flow, doppler).
A B-mode image, for example, is a brightness image in which component pixels are brightened in proportion to a corresponding echo signal strength. The brightness image represents a cross section of a patient target area through a transducer's scanning plane. Typically the B-mode image is a gray scale image in which the range of darker to lighter gray-scale shades corresponds to increasing brightness or echo strength.
One common ultrasound application is to view a fetus within the mother's womb during a prenatal care stage. It is desirable to view the fetus to diagnose its health, sex, and age. Fetal head measurements, the subject of this invention, are useful for evaluating fetal growth, estimating fetal weight, predicting intra-uterine growth retardation and fetal maturity, and estimating gestational age. The fetal head measurements of interest include fetal head circumference ("HC") and fetal biparietal diameter ("BPD"). The biparietal diameter is the distance between the two parietal bones of the skull at points just above the ears.
Conventionally, these measurements have been performed manually by a sonographer. Head circumference is measured by plotting minor and major ellipse axes on the image, then calculating circumference based upon such axial distances. Thus, the fetal head head is assumed to be an ellipse. The ellipse circumference serves as an estimate of the fetal head circumference. Biparietal diameter is measured using a caliper to read the distance on the fetal ultrasound image. One shortcoming of this conventional manual measurement approach is that the time used in performing the measurements is a large portion of the overall fetal ultrasound scanning process time. Another shortcoming is that the manual measurements introduce operator inaccuracies which impact related and subsequent diagnoses. Some efforts to reduce the time requirement have added to the inaccuracies.
For example, by opening an ellipse to estimate head circumference an inaccuracy is introduced. Specifically, because the head is not likely to be a perfect ellipse, the circumference has a degree of inaccuracy. Because the head circumference is used to derive other parameters and values, there is a degree of inaccuracy throughout the diagnostic evaluation. Even more problematic is that the inaccuracy is not consistent. Different operators may introduce different degrees of inaccuracy. Thus, it is difficult to obtain a meaningful profile of statistics for a given population of data. For example, growth curves used for aging the fetus have been derived from data having varying degrees of inaccuracies decreasing the reliability of the diagnosed age. Reliability of other estimates such as computation of head-to-abdominal circumference ratios and estimation of fetal weight also is reduced. An error in estimating biparietal diameter by about 0.8 mm results, for example, changes estimated gestational age by about one-half week. An error in estimating head circumference by about 7 mm changes estimated gestational age by about one full week. Variability of fetal head size measurements among different operators often ranges between 2% and 7%. As a result, estimated gestational age derived from manual measurements of different sonographers have varied by more than one week. Accordingly, there is a need for a more accurate, more consistent method for measuring fetal head size.